Superconductors are materials having essentially zero resistance to the flow of electrical current at temperatures below a critical temperature, T.sub.c. A variety of copper oxide materials have been observed to exhibit superconductivity at relatively high temperatures, i.e., above 77 K. Since the discovery of the copper oxide-based superconductors, their physical and chemical properties have been widely studied and described in many publications, too numerous to be listed individually.
Composites of superconducting materials and metals are often used to obtain better mechanical and electrical properties than superconducting materials alone provide. These composites may be prepared in elongated forms such as wires and tapes by a well-known process which includes the stages of: (a) forming a powder of superconductor precursor material (precursor powder formation stage); (b) filling a metal container, such as a tube, billet or grooved sheet, with the precursor powder and deformation processing one or more filled containers to provide a composite of reduced cross-section including one or more cores (filaments) of superconductor precursor material in a surrounding metal matrix (composite precursor fabrication stage); and (c) further thermally processing the composite to form and sinter a core material having the desired superconducting properties (thermomechanical processing).
In order to be useful for the majority of applications, substantially single phase superconducting materials with high critical current densities (J.sub.c) are needed. The current-carrying capacity of a superconducting oxide depends significantly upon the degree of alignment and connection of the superconducting oxide grains, together known as "texturing". The processing of such high-performance superconducting materials may be constrained by the necessity of texturing and densifying the material in order to achieve adequate critical current density. The need to add additional processing steps into the manufacture of an oxide superconductor article in order to achieve adequate critical current density adds significantly to the cost of the final article.
Known processing methods for texturing superconducting oxide composites include various forms of heat treatments and deformation processing (thermomechanical processing). Certain superconducting oxide grains can be oriented along the direction of an applied strain, a phenomenon known as deformation-induced texturing (DIT). Deformation techniques, such as pressing and rolling, have been used to induce grain alignment of the oxide superconductor c-axis perpendicular to the plane or direction of elongation. Heat treatment under conditions which at least partially melt and regrow desired superconducting phases also may promote texturing by enhancing the anisotropic growth of the superconducting grains, a phenomenon known as reaction-induced texturing (RIT).
Typically, density and degree of texture are developed in the article by repeated steps of deformation (to impart deformation-induced texturing) and sintering (to impart reaction-induced texturing). The steps of deforming and sintering may be carried out several times, resulting in a process that is both time consuming and expensive. The process may be designated by the term "nDS", in which "D" refers to the deformation step, "S" refers to the sintering or heating step and "n" refers to the number of times the repetitive process of deformation and sintering are carried out. Typical prior art processes are 2DS or 3DS processes.
It is desirable, therefore, to provide a method for preparing a superconducting article having critical current densities acceptable to the art in fewer steps. It is desirable to minimize the number of process steps required while obtaining an acceptable degree of texture and oxide density. In particular, it is desirable to prepare such a superconducting article using a simplified deformation-sintering process which reduces the amount of processing steps required to obtain a superconducting oxide article having adequate critical current. In determining adequate critical current, price to performance ratio ($/KA.multidot.m) should be minimized.
The prior art has investigated many texturing processes, but have been unable to reduce the deformation-sintering process to a single iteration, that is a 1DS process while retaining acceptable electrical properties. Chen et al. in U.S. Pat. No. 5,208,215 reports a method of gradually reducing the thickness of a superconducting oxide tape through at least two pressing steps, each pressing step followed by a heating step at 843.degree. C., thus reporting at least a 2DS process. Sumitomo Electric Industries, in New Zealand Application No. 230404, reports on 2DS processes for improving high critical current density, in which a variety of processing conditions such as sintering temperatures and times, cooling rates, percent deformation and deformation loads were investigated.
Hikata et al. in U.S. Pat. No. 5,246,917 reports the use of a 2DS or 3DS process for improving critical current density of an oxide superconducting wire which utilizes second rollers for the second rolling process which are at least 5 cm larger than those used in the first rolling step. Sumitomo Electric Industries in EP 0 504 908 A1 reports a 2DS process in which the rolling operation is carried out with rollers having an increased frictional force.
Likewise, Sumitomo Electric Industries, in EP 0 435 286 A1 ("EP '286"), reports a two-step rolling operation for a monofilament wire. Large reduction rolling drafts are used to form a flattened tape, however, EP '286 discloses that a subsequent rolling operation at smaller drafts is performed before the final heat treatment, thereby teaching a 2DS process. Further, the thicknesses of monofilament wires make it difficult to obtain crack-free high performance wires in a single rolling operation.
Some 1DS processes have been reported; however, they do not result in oxide superconducting wire having acceptable critical current densities. Takahashi et al. in U.S. Pat. No. 5,093,314 disclose a perovskite oxide superconductor monofilament wire prepared by subjecting the wire to repeated extrusions, followed by rolling and heat treatment (a 1DS process) However, Takahashi et al. report performing deformation operations on fully formed oxide superconductor powders which are known to respond poorly to RIT and DIT processes. The fully formed oxide superconductor is not well suited to RIT because the product oxide has already been formed. For example, critical current densities reported by Takahashi et al. are no greater than 3800 A/cm.sup.2 for a BSCCO sample.
In a similar fashion, Sumitomo Electric Industries, in EP 0 281 477 A2, describes a process for manufacture of an oxide superconductor in which the oxide superconductor powder is first formed and then introduced into a metallic tube and plastically deformed under compressive strain. Such a method suffers from limitations similar to those mentioned above for Takahashi et al.
Common deformation techniques in nDS processes include extrusion, drawing, rolling or pressing. Common measures of the effectiveness of the deformation process are expressed as degree of texture, core hardness and core density. Increased core hardness has been associated with improved texturing and core density. Core hardness is a measurement of the hardness of the material as determined by a standard test, such as an indent test. Core density is the density of the oxide powder. Degree of texturing is represented by a fraction between one and zero, with one representing 100% alignment of the c-axes of the oxide grains, such that their slip planes are parallel.
Uniaxial pressing may be an effective method of both aligning the anisotropic oxide grains and densifying the oxide core. See, Li et al. Physica C 217, 360-366 (1993) and Korzekawa et al. Appl. Superconduct. 2(3/4), 261-270 (1994), incorporated by reference herein. The pressing technique has at least one serious drawback in that it can not be carried out conveniently, uniformly and continuously over long lengths of superconducting material.
Rolling, on the other hand, is well-suited for continuous processing of long lengths of superconducting material, particularly wire or tape. However, the rolling operation may sometimes induce cracks and shear bands of the oxide filaments in a direction disruptive to current flow. Further, certain rolling conditions lead to the undesirable distortion of the oxide/metal interface, known as "sausaging". See, Li et al. This type of deformation is illustrated in FIG. 1, in which dark regions 10 represent oxide filaments and lighter regions 12 represent a surrounding metal matrix. Under certain rolling conditions, an interface 14 of the composite is distorted into a rolling, wavy conformation, resulting in alternating narrow regions 16 and wide regions 18 in the oxide filaments 10.
Cracking, formation of shear bands and sausaging may be reduced in a rolling process by the use of extremely low reduction drafts. Typically, multiple reductions on the order of 3% to 5% per pass are used. See, Korzekwa et al., Id. While such low reduction passes may reduce sausaging, it has not been shown to eliminate it entirely. Additionally, low reduction passes exert only a small working force on the composite so that core density, texture and hardness remain very low. For example, the degree of texturing is only around 0.6. Also important, low reduction drafts require multiple processing steps to obtain suitably thin dimensions, thereby increasing the chances of material mishandling and driving up processing costs.
EP 0 435 286 by Sumitomo Electric Industries (EP '286) describes the preparation of monofilament superconducting oxide wire by flat working the wire at a draft of 80% to 98% reduction in thickness with rollers of 150 mm (7") diameter, followed by subsequent heat treatment and low reduction rolling. EP '286 reports an increase in critical current density and suggests that it may be due to improvements in density and orientation, but no mention is made of cracks or rolling defects, such as sausaging. Investigation by applicants has established that roll working of monofilaments is less sensitive to sausaging. Therefore, EP '286 does not address the sausaging problem in multifilamentary systems.
It has been observed that preferred alignment of superconducting oxide grains can be obtained by the growth of oxide grains in long, thin filaments constrained within a metal matrix. The application of constrained growth of oxide grains is set forth in greater detail in U.S. Ser. No. 08/059,871 filed May 19, 1993, which is a file wrapper continuation of U.S. Ser. No. 07/686,792, now abandoned, entitled "A Method of Producing Textured Superconducting Oxide Bodies by the Oxidation/Annealing of Thin Metallic Precursors", which are incorporated by reference in its entirety herein. The application describes superconducting oxide compositions having improved crystallographic alignment by constraining the oxide filament diameter to a dimension on the order of the longest dimension of the oxide superconductor grain. It is therefore desirable to prepare superconducting articles having such constrained dimensions so as to take advantage of constrained grain growth.
As discussed extensively above, however, deformation rolling introduces structural defects or sausaging which destroy the constrained volume, at least locally (see, FIG. 1). When rolling operations are carried out on compositions containing such long, thin filaments, "sausaging" occurs which results in localized filament diameters which are considerably greater than desired. In those regions, texturing and core density is reduced, as described above. FIG. 2 schematically illustrates the effect that the narrowing and expansion of the filament thickness along the filament length has on the alignment of grains of a constrained multifilamentary oxide superconducting article 40. Article 40 includes regions 42 having a dimension sufficient to constrain grain growth to substantially parallel to the constraining dimension, so as to provide dense, aligned oxide grains 44. Article 40 also contains regions 46 which are of significantly greater dimension. Region 46 is typically introduced into the article 40 in rolling deformation processes. The dimension of region 46 is too large to constrain grain growth to a particular orientation. As a result, oxide grains 48 are randomly oriented. Porosity of region 46 increases significantly because of the reduced packing efficiency of the randomly oriented grains.
Therefore, there remains a need to develop deformation processes that avoid cracking, shear band formation and sausaging, while imparting the desired degree of texturing, core density and hardness to a multifilamentary superconducting oxide article. Multifilamentary articles are more useful than monofilamentary articles for mechanical properties and AC applications. This invention enables high-performance multifilamentary oxide superconductor articles having superior performance as compared to prior art mono- and multifilamentary systems.
It is an object of the present invention to reduce the number of iterative deformation-sintering steps in the processing of an oxide superconductor article.
It is an object of the present invention to provide a method for forming a superconducting article having fine and uniform superconducting oxide filaments without cracks, band shear or sausaging.
It is a further object of the present invention to provide a method for preparing a superconducting oxide article having uniformly high degree of texturing and density using a high reduction deformation process.
It is a further method of the present invention to provide a method for deforming a multifilament superconducting article which optimizes the degree of texturing and core density in a minimum of processing steps.
It is an object of the present invention to provide a method for preparing a superconducting article having high critical current densities using fewer processing steps, namely a high reduction deformation step immediately preceding a single sintering process.